A gas turbine engine includes a turbomachinery core having a high pressure compressor, combustor, and high pressure turbine (“HPT”) in serial flow relationship. The core is operable in a known manner to generate a primary gas flow. The high pressure turbine includes annular arrays (“rows”) of stationary vanes or nozzles that direct the gases exiting the combustor into rotating blades or buckets. Collectively one row of nozzles and one row of blades make up a “stage”. Typically two or more stages are used in serial flow relationship. These components operate in an extremely high temperature environment, and must be cooled by air flow to ensure adequate service life.
HPT nozzles are often configured as an array of airfoil-shaped vanes extending between annular inner and outer bands which define the primary flowpath through the nozzle.
Due to operating temperatures within the gas turbine engine, it is desirable to utilize materials with low coefficient of thermal expansion. For example, to operate effectively in such strenuous temperature and pressure conditions, composite materials have been suggested and, in particular for example, ceramic matrix composite (CMC) materials. These low coefficient of thermal expansion materials have higher temperature capability than metallic parts. The higher operating temperatures within the engine result in higher engine efficiency. However, such ceramic matrix composite (CMC) have mechanical properties that must be considered during the design and application of the CMC. CMC materials have relatively low tensile ductility or low strain to failure when compared to metallic materials. Also, CMC materials have a coefficient of thermal expansion which differs significantly from metal alloys used as restraining supports or hangers for CMC type materials. Therefore, if a CMC component is restrained and cooled on one surface during operation, stress concentrations can develop leading to failure of the segment.
Prior art nozzles formed of CMC materials have been attempted with limited success. These nozzles must have constructions wherein load controlled stresses are minimized. Attempts have been made to carry pressure loads acting on the CMC nozzle to support at the outer and inner bands of the nozzle. Generally, moments are created at the fillets of the inner and outer bands to accomplish this construction. This results in high stresses at the interfaces of the vanes and bands, creating durability challenges for the CMC components.
It would be desirable to improve known nozzle assemblies in order to eliminate the creation of moment at the interface between the nozzle and associated attachment features. It would further be desirable to provide an assembly to support the CMC nozzle while limiting load on the part. It would further be desirable to allow for differential thermal growth between parts of differing material types.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded subject matter by which the scope of the invention is to be bound.